So, what does “as needed” mean? Shouldn’t all abbreviations and acronyms be defined?

Almost, but there are a handful of exceptions. These exceptions are words for which the abbreviated forms have become commonplace. These abbreviations are often better known than their spelled-out counterparts. IQ, for example, is better known than is intelligence quotient. Likewise, your readers are more likely to recognize REM sleep than rapid eye movement sleep.

If these seem arbitrary, don’t worry! You won’t have to phone a friend each time you consider using an abbreviation. Just follow these guidelines, as recommended by the Publication Manual (p. 107):

You may use “abbreviations that appear as word entries (i.e., that are not labeled abbr) in Merriam-Webster’s Collegiate Dictionary (2005).” For these few cases, you don’t need to define the abbreviations.

Conversely, entries that do include the abbr label are abbreviations that should be defined.

If an abbreviation does not appear in the dictionary, you should define it.

Finally, see pages 106–108 of the Publication Manual for additional guidance on abbreviations, including how and when to introduce them, examples of under- and overuse, and more.